In the field of hydrocarbon exploration, measurements are often made on reservoir fluids. Such measurements are typically made to obtain information on different reservoir fluid properties, e.g., resistivity, NMR, optical absorption and scattering, dielectric constant, etc. Measurements may be made on fluids in the formation or in a flowline of a fluid sampling tool. Fluid sampling tools are widely used in the well-logging industry. Borehole fluid sampling tools have one or more probes that are pressed against the borehole wall so that reservoir fluids can be pumped out of the earth formation into a flowline or sample bottle situated in the fluid sampling tool. A sample placed in a sample bottle may be preserved so that it may be tested in a laboratory (i.e., a “live oil” sample).
Fluid sampling tools may be used to measure reservoir pressures. Those tools often operate in high pressure and high temperature environments. The pressure of reservoir fluids in a flowline can exceed 25,000 pounds per square inch (psi), and temperatures can approach or even exceed 200° C. Because of the high temperatures and pressures, the flowlines used in commercial fluid sampling tools are typically made of stainless steel or some other suitable metal alloy.
A commercially available, NMR-compatible pressure cell for laboratory use exists in the prior art and is frequently used in conjunction with a 2 MHz laboratory NMR instrument to perform NMR measurements on fluid (e.g., oil) samples at elevated temperatures and pressures. In the laboratory apparatus, the NMR radio frequency (RF) coil used to excite the protons in the reservoir fluids is situated outside of the pressure cell. The pressure cell is made of a non-conductive and non-magnetic plastic. The internal pressure on the inner walls of the pressure cell is at least partially offset by a pressurized, NMR-insensitive fluid contained outside the cell. The pressurized fluid reduces the net pressure on the inner walls of the pressure cell, and can also be heated to regulate the temperature of the sample. This type of pressure cell is not, however, viable for use in a downhole fluid sampling tool because it is incapable of operating at the elevated temperatures and pressures encountered in a wellbore.
The RF antenna used in the above pressure cell is a solenoidal coil that is located outside of the pressure cell and encircles the sample. The fluid sample only partially occupies the interior region of the RF coil. This results in a reduced “fill factor”, in this case on the order of 0.3. The low fill factor is problematic because the signal-to-noise ratio (S/N) of the NMR measurement is directly proportional to the fill factor. Another problem with the prior art pressure cell is that it requires pressure compensation, as described above, which adds to the complexity, cost, and maintenance of the tool. One apparatus using such a pressure cell has maximum pressure and temperature ratings of 10 Kpsi and 260° F., respectively. Those maximum ratings are far too low for modern fluid sampling logging tools that must be able to analyze fluids at temperatures and pressures in the neighborhood of 30 Kpsi and 400° F., respectively.
U.S. Pat. No. 4,785,245 (the '245 patent) issued to Lew et al. discloses a NMR device for monitoring the fraction of oil in a multi-phase flow stream coming from a producing oil reservoir. The '245 patent discloses a non-metallic flow pipe that has a RF receiver coil and a separate transmitter antenna, both mounted on the outer surface of the flow pipe. The flow pipe is made from ceramic or other non-conductive and non-magnetic material. The NMR magnet, flow pipe, and associated antennas disclosed in the '245 patent, however, are not capable of making NMR diffusion measurements, and are not suitable for the high pressure and temperature environment encountered by borehole fluid sampling tools.
Prammer et al., in U.S. Pat. No. 6,737,864 (the '864 patent) issued to Halliburton Energy Services, Inc., discloses a NMR device for use in a fluid sampling logging tool. The '864 patent uses a flow tube made from ceramic, glass, or plastic. Those materials do not have the strength to withstand the pressures encountered in typical downhole environments. Moreover, the RF coil is outside of the flow tube, which limits the S/N of the measurement, and there are no gradient coils for Pulsed Field Gradient (PFG) diffusion measurements.
Kleinberg, in U.S. Pat. No. 6,346,813 (the '813 patent, assigned to the assignee of the present invention), discloses a NMR module for a fluid sampling logging tool. Kleinberg recognized that a metal flowline attenuates electromagnetic (EM) radiation from antennas or other transmitters situated outside of the flowline. The attenuation caused by metallic or highly conductive steel flowlines causes severe signal-to-noise ratio problems for measurements made by EM sensors situated outside of the flowline. The '813 patent discusses, among other things, a metal flowline with a RF coil located in the interior region of the flowline. The flowline is enclosed by a permanent magnet; specifically, a circular Halbach magnet configuration that surrounds the flowline. However, certain features of the present invention are not disclosed by Kleinberg. For example, the '813 patent does not teach that the PFG coils can be advantageously located outside of the metal flowline without comprising diffusion measurements.
The '864 patent and U.S. Pat. No. 6,111,408 issued to Blades and Prammer disclose two types of magnet assemblies wherein the magnet surrounds the flowline. Both patents disclose a circular Halbach magnet design that consists of an array of eight magnet pieces surrounding the flowline. The magnetization direction of each magnet piece is different, i.e., it increases by 90° going clockwise from one piece to the next. In theory, this design produces a relatively homogeneous magnetic field over the measurement region. However, in practice it is difficult to manufacture this magnet and small deviations of the directions of the magnetization or field strength from those prescribed by theory can result in a substantial gradient in the magnetic field in the region of the flowline. The '864 patent also discloses a circular array of eight magnetic dipoles that surround the flowline.
The magnet designs disclosed in the prior art patents on NMR magnets for borehole fluid sampling and pressure tools have several shortcomings. For example, prior art magnet designs that have magnets surrounding the flowline do not provide space for “thru-wires”. Thru-wiring is required for combinability of a NMR measurement module with other measurement modules. Moreover, prior art designs do not provide for continuous flow within or through the sampling tool when NMR measurements are desired on a stationary sample. For example, NMR diffusion measurements are preferably made on fluids that are stationary (non-flowing). In such instances, the prior art devices require the flow (pumping) be stopped while NMR measurements are performed. By having separate flowlines for the sample and through-put fluids, through-put fluids can be pumped through a flowline without interruption while acquiring NMR data on stationary fluids in a NMR measurement flowline.